1. Field of the Invention
The present invention relates to an extreme ultra violet (EUV) light source apparatus to be used as a light source of exposure equipment.
2. Description of a Related Art
Recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography requiring power of more than several tens of watts.
Here, a principle of generating EUV light in the LPP type EUV light source apparatus will be explained. When a laser beam is applied to a target material supplied into a vacuum chamber, the target material is excited and plasmarized. Various wavelength components including EUV light are radiated from the plasma. Then, the EUV light is reflected and collected by using an EUV collector mirror that selectively reflects a desired wavelength component (e.g., a component having a wavelength of 13.5 nm), and outputted to an exposure unit.
FIG. 5 shows an example of the conventional EUV light source apparatus. As shown in FIG. 5, the EUV light source apparatus 61 includes a vacuum chamber 62 in which EUV light is generated, a target supply unit 64 for supplying a target 63 to a predetermined position within the vacuum chamber 62, a driver laser 66 for generating an excitation laser beam 65 to be applied to the target 63, a laser beam focusing optics 67 for focusing the excitation laser beam 65 generated by the driver laser 66, and an EUV collector mirror 70 for collecting and outputting EUV light 69 emitted from plasma 68 generated when the excitation laser beam 65 is applied to the target 63, a magnetic field generating unit including a magnetic field coil 77 for generating a magnetic field that confines ionized debris included in debris generated from the plasma 68, and an evacuating pump 80 for evacuating the vacuum chamber 62.
In the LPP type EUV light source apparatus, there is a problem that the debris emitted from the plasma 68 attach to the surfaces of the optical elements of the EUV collector mirror 70, the laser beam focusing optics 67, a laser beam entrance window 74, an SPF (spectral purity filter) (not shown), an entrance window of an optical sensor (not shown), and so on, and reduce the reflectance or transmittance of EUV light, and thereby, reduce the output of EUV light and/or sensitivity of the sensor. In order to solve the problem, a technology of confining and ejecting the ionized debris generated from plasma by using a magnetic field to the outside of the vacuum chamber is known (Japanese Patent Application Publication JP-P2005-197456A). The debris refers to flying materials from plasma including neutral particles and ions and waste target materials.
For example, when the tin metal target 63 is excited by the excitation laser beam 65, most of tin becomes plasma 68 including polyvalent positive ions and electrons. When a magnetic field is applied to the region including the plasma 68 by the magnetic field coil 77, the positive tin ions are constrained by the magnetic field and moved in a direction along lines of magnetic force. Thereby, the amount of the positive tin ions attaching to the optical elements of the EUV collector mirror 70, the laser beam focusing optics 67, the laser beam entrance window 74, the SPF (not shown), the entrance window of the optical sensor (not shown), and so on is reduced, and the positive tin ions are efficiently ejected to the outside of the vacuum chamber 62 by the evacuating pump 80.
JP-P2005-197456A discloses protection of the EUV collector mirror by trapping the ionized debris included in the debris generated from the plasma by using the magnetic field within the vacuum chamber of the EUV light source apparatus.
Further, Japanese Patent Application Publication JP-P2006-210157A discloses generation of EUV light by cooling and pressurizing tin hydride (SnH4) to release the tin hydride in droplets or liquid jet and plasmarizing the tin hydride by using a laser beam.
As explained above, in the conventional LPP type EUV light source apparatus, since the ionized debris are constrained by the magnetic field and moved in the direction of lines of magnetic force and efficiently ejected by the evacuating pump, the ionized debris can be prevented from attaching to the optical elements within the chamber to deteriorate the performance of the EUV light source apparatus.
However, part of the generated positive ions recombine with electrons and become neutral particles, move without being constrained by the magnetic field, and attach to the surfaces of the optical elements within the chamber to reduce the reflectance and transmittance of EUV light, and thereby, reduce the performance of the EUV light source apparatus. Especially, polyvalent positive ions of tin or the like easily recombine with electrons and reduce the performance of the EUV light source apparatus.
Further, it is difficult to ionize all target materials by the excitation laser, and part of the target materials become neutral debris, move without being constrained by the magnetic field, and attach to the optical elements within the chamber to reduce the reflectance or transmittance of EUV light, and thereby, reduce the performance of the EUV light source apparatus.